Acidity of Carboxylic Acids

B.Sc. Part 2 Organic Chemistry – Paper 2

Dr. Jyotsna Chaturvedi

HCPG College, Varanasi

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Acidity of Carboxylic Acids Carboxylic acids are a class of organic compounds which contain at least one

carboxyl group (-COOH) as a functional group. Acids containing one, two or

three carboxyl groups are known as mono ,di and tricarboxylic acids

respectively. They are obtained by replacement of a hydrogen atom from an

aliphatic or aromatic hydrocarbon and are represented by the general formula

RCOOH or ArCOOH respectively. Whether the group is aliphatic or aromatic,

saturated or unsaturated, substituted or unsubstituted the properties of the

carboxyl group are essentially the same. However, the nature of R and Ar may

increase or decrease the acidity of carboxylic acids.

In aqueous solution a carboxylic acid exists in equilibrium with the carboxylate

anion and the hydrogen ion:

RCOOH + H2O ⇌ RCOO—

+ H3O+

and,

Since water is the solvent, its concentration remains constant. Therefore,

Where Ka is called acid dissociation constant or acidity constant and is equal

to Keq [H2O]. Each carboxylic acid has its characteristic value for Ka which

indicates its strength. Higher is the value for Ka stronger is the acid and vice

versa. A higher value for Ka indicates better ionisation of RCOOH or Ka is

proportional to concentration of hydronium ion or concentration of hydrogen

ion.

The strength of an acid is represented by the negative logarithm of Ka (pKa).

pKa is very much similar to pH or

pKa = - log10 Ka

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A strong acid has a low value of pKa whereas a higher value of pKa indicates

that the acid is weak. Value of pKa for Formic acid is 3.77, for Acetic acid it is

4.74 and for trichloroacetic acid it is 0.65.

The Bronsted-Lowry acid base theory defines acids (HA) as species that lose

hydrogen ions and form a conjugate base. A strong acid loses its hydrogen ion

easily and forms a stable conjugate base. In case of carboxylic acids loss of

hydrogen ion is easier due to conjugation between lone pair of electron of

Oxygen of --OH and pi electrons:

The delocalisation of electrons creates a positive charge on Oxygen atom

making it more electronegative. This weakens the hydroxyl bond and loss of

hydrogen ion becomes easier. The loss of hydrogen ion generates a carboxylate

anion (C&D):

The anion is resonance stabilised and structure C & D contribute equally.

Therefore, carboxylate anion serves as a stable conjugate base.

All those factors(in R/Ar) which increase the positive charge of structure B

and decrease the negative charge of structure C & D will increase the

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acidity and the factors which decrease the positive charge of structure B

and increase the negative charge of structure C&D decrease the acidity.

Effects of substituents on acidity of carboxylic acids

Aliphatic Acids: Aliphatic acids are represented by general formula RCOOH.

R may be a hydrogen atom (HCOOH, Formic acid) or an alkyl group or an alkyl

group with electron donating substituents or an alkyl group with electron

withdrawing substituents. HCOOH has a pKa value of 3.77 whereas acetic acid

(CH3COOH) and chloroacetic acid (ClCH2COOH) have pKa values of 4.74 and

2.86 respectively. These values indicate that a methyl group is an electron

donating group which decreases positive charge in structure B and increases the

negative charge in structure C and D making acetic acid less acidic than

formic acid. Greater is the +I effect of alkyl group, weaker is the acid. On the

other hand, Cl of chloroacetic acid increases the positive charge of structure B

and decreases negative charge of structure C and structure D due to its --I effect.

This results in increased acidity of chloroacetic acid. This can be further

confirmed by pKa value of 0.65 for trichloroacetic acid (Cl3CCOOH).

Trichloroacetic acid has three Cl atoms which create a strong –I effect. Acidity

of -haloacids decreases in the decreasing order of electronegativity of

halogens. For example, for FCH2COOH, ClCH2COOH, BrCH2COOH and

ICH2COOH, the pKa values are 2.57, 2.86, 2.90 and 3.16 respectively. For

F3CCOOH it is 0.23.

Aromatic Acids: The simplest member of this group is Benzoic acid

(C6H5COOH). Carboxyl group is directly attached to phenyl ring. Phenyl ring

has a +R effect as well as -I effect. +R effect overcomes the -I effect and net

result is electron donation (+I effect) by phenyl ring to carboxyl group. This

effect is acid weakening and therefore Benzoic acid is weaker (pKa = 4.2) than

formic acid (HCOOH).

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Methyl group of acetic acid (pKa = 4.74) has a greater +I effect than (+R and -I)

effect of phenyl ring of Benzoic acid. Therefore, acetic acid is weaker than

Benzoic acid. Phenylacetic acid (C6H5CH2COOH) (pKa = 4.31) is stronger than

acetic acid because phenyl ring has a -I effect. +R effect of phenyl ring in

phenyl acetic acid is absent as phenyl ring and carboxyl group are separated by

methylene group.

Effects of substituents on acidity of substituted

Benzoic acids

Electron donating substituents decrease the acidity where electron withdrawing

substituents increase the acidity of substituted Benzoic acids. However, the

acidity varies with the position of the substituents. Decrease or increase in acid

strength is more pronounced when electron releasing/withdrawing substituents

are at para position. Substituents at meta position affect the acidity to a lesser

degree. All ortho substituted Benzoic acids are stronger than Benzoic acids

irrespective of the nature of substituents. This is known as Ortho Effect and is

probably due to combination of steric and electronic factors.

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Substituents pKa Values

para meta ortho

-CH3 4.4 4.3 3.9

-OCH3 4.5 4.1 4.1

-OH 4.6 4.1 3.0

-Cl 4.0 3.8 2.9

-NO2 3.5 3.4 2.2 pKa Values of substituted Benzoic acids

-CH3, -OCH3 and -OH are electron releasing substituents and therefore

substituted benzoic acids with these substituents at para position are weaker

than benzoic acid.

Substituted benzoic acids having hydroxy and alkoxy substituents at meta and

ortho position show anomalous behaviour. They are stronger acids than Benzoic

acid. -OCH3 and -OH have both resonance and inductive effects. At para

position resonance effect (electron releasing effect) outweighs the inductive

effect (electron withdrawing effect) and so para-hydroxy and para-methoxy

benzoic acids are weaker than benzoic acid. At meta position inductive effect is

operating and therefore meta-hydroxy and meta-methoxy benzoic acids are

stronger benzoic acids.

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p-chlorobenzoic acid is stronger acid than benzoic acid but weaker than m-

chlorobenzoic acid because inductive effect of Cl is more effective when it is at

meta position in comparison to when Cl is at para position (Inductive effect

decreases with distance). p-nitrobenzoic acid is stronger acid than p-

chlorobenzoic acid. This is because in p-nitro benzoic acid, nitrogen of nitro

group is positively charged and hence electron withdrawing effect of positively

charged nitro group is more. Also, in p-chlorobenzoic acid non-bonding

electron of -Cl increase the electron density (acid weakening) of ring.

Ortho Effect: All ortho substituted benzoic acids are stronger acids than

benzoic acid no matter whether the substituent is electron releasing (-CH3, -

OCH3, -OH, -NH2) or electron withdrawing (-Cl, -NO2, -CN, -COOR etc.). This

effect is known as Ortho effect. It is mainly due to steric effect and if inductive

effect is also favourable, acidity increases to a greater extent. For example,

methyl group has +I effect and therefore, p-methyl and o-methyl benzoic acids

have pKa values of 4.4 and 3.9 respectively, indicating that o-methyl benzoic

acid is stronger than benzoic acid and in this case steric effect (for ortho effect)

overweighs the inductive effect. The difference between pKa values of p-chloro

and ortho-chloro benzoic acid is larger than that of methyl benzoic acids. p-

chloro benzoic acid has a pKa of 4.0 and that of o-chloro benzoic acid is 2.9. In

this case ortho effects involves both steric and inductive effect. If there is

possibility of hydrogen bonding between carboxylate anion and ortho

substituent, acidity increases to a large extent. For example, salicylic acid forms

a six membered ring by intramolecular Hydrogen bond.

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Therefore, anion of Salicylic acid is more stable than anion of p-hydroxy

benzoic acid or m-hydroxy benzoic acid. If there are two hydroxy groups on

both sides of carboxylate anion, stabilisation of anion further increases.

The much greater strength of o-nitrobenzoic acid (pKa = 2.2) as compared to its

meta (pKa = 3.4) and para isomer (pKa = 3.5) is the result of intermolecular H-

bonding.

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In an ortho-substituted benzoic acid, the ortho substituents prevent the co-

planarity between carboxyl group and the ring. Decrease in coplanarity

diminishes resonance between ring and carboxyl group and it results in

increased positive charge on oxygen of -OH group (structure B) or loss of

Hydrogen becomes easier or acidity increases. If the resulting anion (structure C

& D) is further stabilized by intramolecular Hydrogen bonding, acidity

increases to a greater extent.

Acidity of Dicarboxylic acids

Aliphatic dicarboxylic acids like oxalic acid (HOOC-COOH), malonic acid

(HOOC-CH2-COOH), succinic acid (HOOC-CH2-CH2-COOH), Glutaric acid

(HOOC-(CH2)3-COOH), and Adipic acid (HOOC-(CH2)4-COOH have pKa

values lower than their monocarboxylic acid analogues. It is reasonable if they

are considered as substituted monocarboxylic acid in which substituent is a

carboxyl group. Carboxyl group is an electron withdrawing group and therefore

acidity of dioic acids will be more than monocarboxylic acids for example:

HOOC-COOH ⇌ HOOC-COO- + H

+ Ionisation of 1

st carboxyl group

HOOC-COO- ⇌

-OOC-COO

- + H

+ Ionisation of

2

nd carboxyl group

In first step loss of H+ is easier due to -I effect of carboxyl group and therefore

pKa1 value of oxalic acid is 1.27. But pKa2 value of oxalic acid is 4.28. This is

because in the second step, loss of H+ occurs from an anion so it is a slow

process and requires a stronger base for removal of Hydrogen ion. As the

distance between two carboxyl groups increases pKa1 increases or acidity

decreases but value of pKa2 decreases showing that loss of Hydrogen ion from

anion becomes easier.

Dicarboxylic acid pKa1 pKa2

Malonic acid 2.80 5.82

Succinic acid 4.20 5.60

Glutaric acid 4.33 5.52

Adipic acid 4.43 5.45

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Acidity of unsaturated dicarboxylic acids

Maleic acid and Fumaric acid are cis and trans forms of Butenedioic acid that is

the two carboxyl groups in these acids are separated by a double bond which

has an electron withdrawing effect. Therefore, they are stronger acids than their

saturated analogue i.e. Succinic acid (pKa1 = 4.20). Maleic acid has pKa1 of 1.92

and Fumaric acid has pKa1 of 3.02. But pKa2 of Maleic acid is 6.23 and pKa2 of

Fumaric acid is 4.38. This can be explained by their configuration. Maleic acid

is cis form of acid and first ionisation is easier than first ionisation of Fumaric

acid (trans-form). In the cis form, the maleate anion is stabilized by

intramolecular hydrogen bonding. This type of stabilisation is not possible due

to trans configuration of Fumarate anion. pKa2 of Maleic acid (6.23) is more

than pKa2 of Fumaric acid (4.38) because stable anion of maleic acid requires a

stronger base for loss of second Hydrogen ion in comparison to anion of

Fumaric acid.

END